It is known that spark plugs, in particular for gas turbines and jet engines, may be of two types:                high-energy high-voltage (HEHV) spark plugs, of which the operating voltage is approximately 20 kV;        high-energy low-voltage (HELV) spark plugs, of which the operating voltage is approximately 2 to 3 kV; these are capable of providing energy of some tenths of a Joule to several Joules.        
HELV spark plugs comprise a material such as a cermet between their electrodes, so that the application of a sufficient voltage between these electrodes results in the passage of a spark.
The behavior of the material during the different phases of the spark has been analyzed as being as follows:
Firstly, the cermet is active during the ionization phase which corresponds to an accumulation of charges on the surface of the material. Then comes the arcing phase which corresponds to an intermediate zone during which there is a step-by-step propagation of micro-arcs on the said surface. Lastly comes the sparking phase during which the semiconductor is inactive but undergoes considerable mechanical and thermal stress caused by the passage of the spark.
It will be noted that in the everyday language of spark plug manufacturers, this material is sometimes described as a “semiconductor”. However, this practice does not truly correspond to actual fact. Indeed, during the arcing phase, the material becomes a conductor on its surface, but not throughout its entire volume.
The advantages of HELV spark plugs are linked, on the one hand, with their operation which depends little on the conditions existing in the combustion chamber (re-ignition under high pressure) and, on the other hand, with the shorter ignition chain which must be created in order to ensure their operation. These are the advantages which have led to the development of silicon carbide-based ceramic materials described in particular in documents U.S. Pat. No. 5,028,346 and FR-A-2 346 881. These materials also include an insulating phase based, for example, on silicon nitride and modified silicon ox nitride, or silica, alumina and alkaline-earth oxide.
However, if these systems have not been generally adopted on engines, it is because the lifetime of a spark plug comprising these materials becomes very short if used under harsh conditions, namely at high pressures and high temperatures combined with chemical attacks linked to the type of fuel used.
In HEHV spark plugs, it is the wear of the electrodes which limits the lifetime of the spark plugs whereas in HELV spark plugs, the ceramic wears out substantially before the electrodes, and it is this that limits their lifetime.
The object of the invention is to increase the lifetime, and therefore the reliability, of HELV spark plugs.